1. Field of the Invention
This invention relates to hydrodynamic rotary seals for bi-directional or uni-directional rotation that are used to retain a lubricant and exclude an environment. More specifically, this invention relates to a feature that improves seal lubrication in adverse conditions such as high operating temperature, skew-resisting confinement, high differential pressure, high initial compression, adverse tolerance accumulation, circumferential compression, high modulus seal materials, dynamic runout, reversing differential pressure, thin viscosity lubricants, third body seal surface wear, and/or material swell (collectively referred to as “severe operating conditions”).
2. Description of the Related Art
The following commonly assigned patent documents represent prior art that is related to the invention:
United States Patents:
U.S. Pat. No. 7,562,878 Low torque hydrodynamic lip geometry for bi-directional rotation seals;
U.S. Pat. No. 7,052,020 Hydrodynamic Rotary Seal;
U.S. Pat. No. 6,767,016 Hydrodynamic Rotary Seal With Opposed Tapering Seal Lips;
U.S. Pat. No. 6,685,194 Hydrodynamic Rotary Seal With Varying Slope;
U.S. Pat. No. 6,561,520 Hydrodynamic Rotary Coupling Seal;
U.S. Pat. No. 6,494,462 Rotary Seal With Improved Dynamic Interface;
U.S. Pat. No. 6,382,634 Hydrodynamic Seal With Improved Extrusion Abrasion and Twist Resistance;
U.S. Pat. No. 6,334,619 Hydrodynamic Packing Assembly;
U.S. Pat. No. 6,315,302 Skew Resisting Hydrodynamic Seal;
U.S. Pat. No. 6,227,547 High Pressure Rotary Shaft Sealing Mechanism;
U.S. Pat. No. 6,120,036 Extrusion Resistant Hydrodynamically Lubricated Rotary Shaft Seal;
U.S. Pat. No. 6,109,618 Rotary Seal With Enhanced Lubrication and Contaminant Flushing;
U.S. Pat. No. 6,036,192 Skew and Twist Resistant Hydrodynamic Rotary Shaft Seal;
U.S. Pat. No. 6,007,105 Swivel Seal Assembly;
U.S. Pat. No. 5,873,576 Skew and Twist Resistant Hydrodynamic Rotary Shaft Seal;
U.S. Pat. No. 5,823,541 Rod Seal Cartridge for Progressing Cavity Artificial Lift Pumps;
U.S. Pat. No. 5,738,358 Extrusion Resistant Hydrodynamically Lubricated Multiple Modulus Rotary Shaft Seal;
U.S. Pat. No. 5,678,829 Hydrodynamically Lubricated Rotary Shaft Seal With Environmental Side Groove;
U.S. Pat. No. 5,230,520 Hydrodynamically Lubricated Rotary Shaft Seal Having Twist Resistant Geometry;
U.S. Pat. No. 5,195,754 Laterally Translating Seal Carrier For a Drilling Mud Motor Sealed Bearing Assembly;
U.S. Pat. No. 4,610,319 Hydrodynamic Lubricant Seal For Drill Bits;
United States Patent Applications:
Pub. No. 2005/0093246 Rotary Shaft Sealing Assembly;
Pub. No. 2006/0214379 Composite, High Temperature, Dynamic Seal and Method of Making Same;
Pub. No. 2009/0250881 Low Torque Hydrodynamic Lip Geometry for Bi-Directional Rotation Seals;
Pub. No. 2007/0013143 Filled Hydrodynamic Seal With Contact Pressure Control, Anti-Rotation Means and Filler Retention Means;
Pub. No. 2007/0205563 Stabilizing Geometry for Hydrodynamic Rotary Seals; and
Pub. No. 2009/0001671 Rotary Seal with Improved Film Distribution.
Assignee Kalsi Engineering manufactures various configurations of hydrodynamic rotary seals, based on the above-referenced patents and patent applications, and sells them under the registered trademark “KALSI SEALS.” The rotary seals that are marketed by Kalsi Engineering are typically installed with radial interference (i.e., compression), and seal by blocking the leak path. Such seals are being challenged to operate at ever-greater temperatures and differential pressures. For general examples of such seals, see FIG. 3 of U.S. Pat. No. 5,230,520, FIG. 4 of U.S. Pat. No. 6,315,302, and FIG. 6 of U.S. Pat. No. 6,382,634.
Upon installation in a compressed condition, hydrodynamic seals define an “interfacial contact footprint” (sometimes just called the “footprint”) that represents the shape of the “dynamic sealing interface,” and the terms are generally interchangeable. Examples of footprints are shown in FIG. 2 of assignee's U.S. Pat. No. 4,610,319 and FIG. 13 of assignee's U.S. Pat. No. 5,230,520. The seals employ various variable width dynamic lip geometries that cause a lubricant-side edge of a dynamic sealing interfacial contact footprint to be wavy. The environment side of the interfacial contact footprint is intended to be substantially circular, to avoid hydrodynamic activity with the environment, and thereby exclude the environment.
As a consequence of the wavy lubricant-side footprint edge, the rotary motion of the lubricant-wetted shaft drags lubricant into the dynamic sealing interface. This hydrodynamic operating regime is intended to allow the seal to operate cooler and with less wear. Although a good level of lubrication is achieved in many cases, in some cases certain designs fall short when exposed to severe operating conditions.
Smaller seal cross-sections are desirable because shaft and housing wall thickness can be maximized. Miniaturization impacts seal lubrication, as described in U.S. Pat. Appl. Pub. 2007/0205563, paragraphs [0036]-[0039]. For a given dimensional compression, interfacial contact pressure increases as a seal cross-section is miniaturized. With radial seals, circumferential compression increases as diameter is miniaturized, increasing footprint spread and contact pressure.
The skew-induced wear mechanism described and illustrated in FIG. 3-27 of the Kalsi Seals Handbook, Rev. 1 is addressed with skew-resisting confinement of the seal, which increases interfacial contact pressure and footprint spread. The term “skew-resisting confinement,” as used herein, encompasses (1) constraint imposed by seal contact with fixed location gland walls as disclosed in U.S. Pat. No. 6,315,302, and (2) spring-loading through a moveable gland wall, as disclosed in U.S. Pat. App. Pub. No. 2009/0001671.
U.S. Pat. No. 6,109,618 teaches the use of abrupt, skewed trailing edge geometries, that are unsuitable as hydrodynamic inlets, on seals suitable only for uni-directional rotation. This abrupt geometry is on the trailing edges of the waves, and is coupled with a very gently converging inlet geometry on the leading edges. Due to the high hydrodynamic leakage of such geometry, and the small reservoir size of downhole tools, such seals cannot be used in downhole oil well applications.
The prior art seals are constructed from elastomers that suffer accelerated degradation at elevated temperature. For example, media resistance problems, gas permeation, swelling, compression set, and pressure related extrusion damage all become worse at elevated temperature. A bi-directional rotation seal that operates with less torque and produces less seal-generated heat would be desirable, in order to moderate such degradation.
U.S. Pat. App. Pub. No. 2009/0001671, “Rotary Seal with Improved Film Distribution” teaches that in the prior art, interfacial lubrication is impaired when the size of a dimensional variable changes due to the effects of certain severe operating conditions. That patent application teaches a lubrication enhancement solution that involves adding more elastomer volume to the seal. This solution is less than perfect in a seal that is axially constrained in accordance with the teachings of U.S. Pat. No. 6,315,302, “Skew Resisting Hydrodynamic Seal,” because the increased seal volume is difficult to accommodate from geometric and interfacial contact pressure standpoints due to the need to accommodate differential thermal expansion between the seal and the hardware it is mounted in. Secondly, the exclusion edge issue disclosed in U.S. Pat. App. Pub. No. 2007/0205563, “Stabilizing Geometry for Hydrodynamic Rotary Seals,” is exacerbated by certain aspects of the U.S. Pat. App. Pub. No. 2009/0001671 solution.
FIG. 1 of U.S. Pat. App. Pub. No. 2009/0001671 is a graph that schematically represents an interfacial contact pressure plot at any circumferential location of a typical seal manufactured according to one of assignee's U.S. Pat. Nos. 4,610,319, 5,230,520, 6,315,302, 6,382,634, and so forth. In that patent application, the labels and dimensions (i.e., first footprint edge L, second footprint edge E, Location P, Dimension A, Dimension B and Width W) are, when necessary, given a subscript “1” or “2” to refer to specific locations of the interfacial contact footprint, width-wise. The portion of the footprint that is circumferentially aligned with Dimension A2 contributes little to overall interfacial lubrication because of lubricant loss at the trailing edge of the wave.
Dimension A2 is related to the size and the shape of the hydrodynamic inlet, and contact pressure at Location P2 is also related to the size and the shape of the hydrodynamic inlet. This precludes independent manipulation of the size of Dimension A2 and the contact pressure at Location P2, and means that the size of Dimension A2 is undesirably large, especially in high temperature operation and/or operation with skew-resisting confinement.
The term “un-swept zone” refers to that portion of the footprint that is circumferentially aligned with Width W1, and the tell “swept zone” refers to the remainder of the footprint. In other words the swept zone is that portion of the footprint that is circumferentially aligned with the footprint wave height. The swept zone is directly lubricated by the sweep of the First Footprint Edge L across the lubricant-wetted shaft. It is a significant undesirable characteristic of the prior art bi-directional rotation seals that lubrication of the un-swept zone is impaired in severe operating conditions.
Elastomers have a high coefficient of thermal expansion. Because there is more material at the widest parts of the dynamic lip, part of the differential thermal expansion between the seal and the housing is relieved circumferentially, causing material displacement from the widest to the narrowest parts of the dynamic lip, and reducing the width of the swept zone while increasing the sizes of Dimension A1 and A2, Dimension B1 and B2, and Width W1 and W2. This effect is exacerbated by skew-resisting confinement.
As the aforementioned sizes increase and the size of the swept zone decreases, lubrication is impaired, causing the seal to generate more and more heat due to increasing asperity friction, and causing a loss of lubricant film viscosity. These factors further increase seal temperature, compounding the problem and leading to an unsustainable runaway operating condition.
Initial compression also causes circumferential compression, which is increased by thermal expansion. Since the seal circumference is relatively long compared to the seal cross-section, circumferential compression can cause buckling in a manner similar to the classic textbook example of a long, slender structural column under compressive loading. This buckling tendency is augmented by the variable stiffness of the prior art seal about its circumference that is caused by the varying dynamic lip width and volume. A seal that has less lip size variation around its circumference would be more desirable than a seal that has more variation, assuming adequate lubrication. Lubricant passing through the region between first footprint edge L2 and Location P2 does little to benefit overall lubrication, because the lubricant film exits at the trailing edge of the wave. Dimension A2 increases significantly when a seal is used in skew-resisting confinement because the footprint spreads as more of the hydrodynamic inlet is brought into contact with the shaft due to seal thermal expansion.
It is desirable to be able to overcome the shortcomings described above. A sealing arrangement that provides a better way to enhance interfacial lubrication would be an advantage in many applications where long sealing life is needed to protect critical components in severe operating conditions.